Hitherto, in order to firmly bond the photographic layer to the plastic film base, two processes have been used, namely, (i) a process which comprises applying the photographic layer directly to the plastic film base after the base is subjected to a surface activating treatment, such as a chemical treatment, an electrical discharge treatment, an ultraviolet ray application, etc., and (ii) a process which comprises applying a subbing layer after carrying out a surface activating treatment and applying the photographic layer to the resulting subbing layer.
Such processes have been described, for example, in U.S. Pat. No. 2,698,241, 2,764,520, 2,864,755, 2,864,756, and 3,475,193 and British Pat. Nos. 788,365, 804,005, and 891,469.
Of these processes, the process (ii) has been more widely used in general. Particularly, in the case wherein the base is polyethylene terephthalate (referred to as PET, hereinafter) or polystyrene, the photographic layer does not sufficiently bond to the base if a subbing layer is not provided after carrying out the surface activating treatment.
However, the above-described surface activating treatment has various faults. For instance, if the surface of the base is treated with a solvent, surface flatness of the base deteriorates and the residual solvent can have an adverse influence upon the photographic layers, and particularly upon the silver halide emulsion layer(s). Furthermore, in order to obtain sufficient adhesion by the ultraviolet ray application, it is necessary to expose the base at a high temperature above the glass transition temperature during the treatment. Consequently, the surface flatness of the base remarkably deteriorates because of decrease of a modulus of elasticity and contraction by heat, etc.
Furthermore, if sufficient adhesion is intended to be obtained by the corona discharge treatment, it is necessary to carry out the treatment at a high temperature above the glass transition temperature, and consequently the same results as in the case of the ultraviolet ray application are obtained.
Moreover, the surface activating treatment not only causes deterioration of external quality but also has an adverse influence upon the photographic layer because of formation of a very small amount of oligomers and acetaldehyde, etc. Accordingly, it has been desired to provide a subbing process by which the sufficient adhesion can be obtained without carrying out the surface activating treatment or with less degree of the surface activating treatment as far as possible, though required.
As one means therefor, there is a process which comprises incorporating a solvent capable of swelling or dissolving the base in a subbing solution.
As known solvents for polyethylene terephthalate, there are, for example, salicylic acid, trichloroacetic acid, pyrrole, chloral hydrate, phenol, p-chlorophenol, pyrogallol and resorcinol, etc.
However, these solvents are generally difficult to handle and toxic to the human bodies. Further, they create problems of environmental pollution. Accordingly, their use is greatly restricted.
For the purpose of overcoming the above-described problems of the prior art, subbing processes using an aqueous dispersion of a polymer latex have been developed in recent times.
Some such subbing processes using an aqueous dispersion of a copolymer composed of (1) a diolefin monomer, (2) one or more vinyl monomers, and, sometimes, (3) another component, are described in Japanese Patent Applications (OPI) Nos. 112326/76, 117617/76, 121323/76, 123139/76 and 139320/76 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application"). Further, the present inventors have studied about a blending process described in Japanese Patent Application (OPI) No. 65422/77 wherein it is intended to obtain desired properties by blending styrene-butadiene based latex with another latex and an addition process described in Japanese Patent Application (OPI) No. 114120/76 wherein it is intended to obtain desired properties by adding additives, e.g., hardeners to styrene-butadiene based latex.
According to these processes, a certain degree of adhesive strength can be obtained without causing problems of toxicity and environmetal pollution. However, in order to obtain sufficient adhesive strength, it is necessary to carry out a separate intensive heat treatment after application and drying in the subbing step. Accordingly, these processes are disadvantageous in both operation and cost.
For the purpose of overcoming this problem, a subbing process has been studied which comprises applying an aqueous dispersion of a copolymer composed of (i) a diolefin monomer and (ii) one or more vinyl monomers, wherein the gel fraction of the copolymer in the aqueous dispersion is less than 80% by weight, as is described in British patent application No. GB 2,040,730A.
According to this process, sufficient adhesive strength can be obtained under a condition of a low heating temperature during the step of application and drying without causing problems of toxicity and environmental pollution. However, in the case wherein the subbing is carried out using a copolymer having a low gel fraction, there is a problem in that the surface of the subbing layer is easily scratched in the drying step. Such a state in the subbing layer can be visually observed through the photographic emulsion layer after application thereof and development of the film, and is therefore very undesirable.